This invention relates to inhalation anesthesia using a closed or semiclosed anesthetic system, and more particularly to the removal of carbon dioxide gas from the exhalation of the anesthetic subject by using an absorbent.
Inhalation anesthesia is a predominant means of a general anesthesia for a surgical operation because inhalation anesthesia has advantages over intravenous anesthesia such as smallness of danger, ease of controlling the depth of anesthesia and earliness of recovery from anesthesia after the operation.
In inhalation anesthesia a portion of the inhaled anesthetic is not metabolized in the body of the patient and remains in the exhalation of the patient. Usually a closed or semiclosed anesthetic system is used to recirculate the exhaled anesthetic with addition of the anesthetic to make up the depletion by the intake in the body.
For the recirculation of the exhaled anesthetic it is necessary to remove carbon dioxide gas contained in the exhalation. Usually an absorbent is used for this purpose, and conventional absorbents for this purpose use calcium hydroxide as the principal material sometimes with the addition of a small amount of an auxiliary component such as barium hydroxide, sodium hydroxide, potassium hydroxide and/or silica. Although any of these conventional absorbents is high in the ability to absorb carbon dioxide gas, there is a possibility that the absorbent adsorbs and decomposes a fraction of the anesthetic coexisting with carbon dioxide gas.
In fact, experiments have evidenced partial decomposition of some widely used inhalation anesthetic by contact with a conventional absorbent for carbon dioxide gas using calcium hydroxide as the principal material. For example, when an experimental exhalation gas containing 2-bromo-2-chloro-1,1,1-trifluoroethane (a widely used inhalation anesthetic, called halothane) and carbon dioxide gas is brought into contact with an absorbent using calcium hydroxide as the principal material in a glass vial, a fraction of halothane is decomposed to form 1,1-difluoro-2-bromo-2-chloroethylene and some other unsaturated compounds. In the case of the contact of fluoromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether (another widely used inhalation anesthetic, called sevoflurane) with calcium hydroxide, a fraction of sevoflurane is decomposed to form 1,1,3,3,3-pentafluoro-2-(fluoromethoxy)-1-propene and/or 1,1,1,3,3-pentafluoro-2-(fluoromethyl)-3-methoxypropane.
It is uncertain whether the above identified decomposition products are harmful or not, and it will take great efforts to clarify the effects of these compounds on the human body. Of course it is undesirable that an inhalation anesthetic contains some substances that have a doubt about pharmacological safety, and even if such a doubt is groundless it is desirable to prevent intrusion of unnecessary substances into an inhalation anesthetic.